Page:EB1911 - Volume 18.djvu/600

 and El ejemplo mayor de la desdicha—are respectively the sources of Rotrou’s Don Bernardo de la Cabrère and Belisaire; Moreto’s Caer para levantar is simply a recast of Mira’s El Esclavo del demonio, a celebrated drama which clearly influenced Calderón when composing La Devoción de la cruz; and there is manifestly a close relation between Mira’s La Rueda de la fortuna on the one hand and Corneille’s Héraclius and Calderón’s En esta vida todo es verdad y todo es mentira. A few of Mira de Amescua’s plays are reprinted in the Biblioteca de autores españoles, vol. xlv.

 MIRAGE (a French word, from mirer, to look at, se mirer, to be reflected), an optical illusion due to variations in the refractive index of the atmosphere. It embraces the phenomena of the visionary appearance of lakes in arid deserts, the images of ships and icebergs, frequently seen as if inverted and suspended in the atmosphere in the Polar Regions, the Fata Morgana, and “looming” as witnessed in mists or fogs.

In the article it is shown that a ray of light traversing a homogeneous medium is deviated from its rectilinear path when it enters a medium of different refractive index; it is therefore readily seen that the path of a ray through continuously varying media is necessarily curvilinear, being compounded of an infinite number of infinitesimally small rectilinear deviations. Our atmosphere is a medium of continuously varying refractive index. Meteorological optical phenomena, due to variations in the refractive index of the atmosphere, may be divided into groups: (1) those due to the permanent or normal variation experienced as one ascends in the atmosphere, and (2) those due to sporadic variations occasioned by irregular heating. The first variation must be taken into account in correcting geodetic observations of heights and astronomical observations of the heavenly bodies; it also has a considerable bearing on the phenomena of the twilight and the afterglow (see : § Astronomical; and ). The second (or temperature) variation gives rise to phenomena which we proceed to discuss.

A common type of mirage is the appearance of an isolated lake frequently seen in hot sandy deserts, as in the Sahara, Turkestan, &c. The explanation is as follows: The sand, being abnormally heated by the solar rays, causes the neighbouring air to expand, consequently its density, and therefore its refractive index, is diminished, and attains a minimum value in the lowest layers. It increases as we ascend and reaches a maximum at a certain height, and then decreases according to the normal variation. Any object viewed across such an area is seen by two sets of rays: one set passing near the earth and assuming a curved path convex to the horizon, the second set more remote from the earth and concave to the horizon. The object thus appears double, an image being seen mirrored in the sand. The sky appears as a shining lake; mountains or palms may be similarly reflected, but it is to be noted that the images are inverted (see fig.). Similar atmospheric conditions sometimes prevail in the air over large, bodies of water on cold autumn mornings. These phenomena have been experimentally realized by R. W. Wood (Phil. Mag., 1899, vol. xlvii.), who viewed objects over a series of heated slate slabs.

Another type of mirage, frequently observed at sea in the northern latitudes, is presented in the appearance of ships and icebergs as if inverted and suspended in the clouds. This is due to a stratum of hot air at some distance above the sea level, the rays of light near the horizon being practically horizontal, while those at greater elevations are fairly concave. It may happen that the change in density is so great that only the upper rays reach the eye; we are then met with the curious illusion of seeing inverted ships in the clouds, although nothing is visible on the ocean.

The Fata Morgana, frequently seen in the Straits of Messina, consists of an apparent vertical elongation of an object situated on the opposite shore. The distribution of density is similar to that attending a desert mirage, but the transition is not so abrupt. The object is really viewed through a horizontally

stratified medium consisting of a central sheet of maximum refractive index, over- and under-laid by sheets of decreasing refractive power. The system consequently acts as a continuous lens, magnifying the object in a vertical direction.

If, in addition to this horizontal stratification, the atmosphere varies similarly in vertical planes, then the object would be magnified both horizontally and vertically. These conditions sometimes prevail in misty or foggy weather, more particularly at sea, and thus give rise to the phenomena known as “looming.” A famous example is the Brockengespenst or “spectre of the Brocken.” The chromatic halos which frequently encircle these images are due to diffraction. (See .) It is interesting to note that lenses formed on non-homogeneous material, having the maximum refractive index along the central axis, have been prepared, and reproduce the effects caused by abnormal distribution of the density of the atmosphere.

The mathematical investigation of this subject was worked out by Gaspard Monge. For this aspect and further details, both descriptive and experimental, see J. Pernter, Meteorologische Optik (1906); E. Mascart, Traité d'optique (1899–1903); R. W. Wood, Physical Optics (1905); R. S. Heath, Geometrical Optics.

 MIRAJ, a native state of India, in the Deccan division of Bombay, forming part of the southern Mahratta Jagirs. Since 1820 it has been subdivided between a senior and a junior branch. The territory of both is widely scattered among other native states and British districts. Area of the senior branch, 339 sq. m.; pop. (1901), 81,467; revenue £23,000; tribute £800. Area of the junior branch, 211 sq. m.; pop. (1901), 35,806; revenue £27,000, tribute £400. The chiefs are Brahmans of the Patwardhan family. The town of, at which the chief of the senior branch resides, is situated near the river Kistna; it is a junction of the Southern Mahratta railway for the branch to Kolhapur. Pop. (1901), 18,425. The chief of the junior branch has his residence at Bhudgaon (pop. 3591).  MIRAMON, MIGUEL (1832–1867), Mexican soldier of French extraction, was born in the city of Mexico, on the 29th of September 1832, and shot with the Emperor Maximilian at Queretaro on the 19th of June 1867. While still a student he helped to defend the military academy at Chapultepec against the forces of the United States; and, entering the army in 1852, he rapidly came to the front during the civil wars. It was largely due to Miramon’s support of the ecclesiastical party against Alvarez and Comonfort that Zuloaga was raised to the presidency; and in 1859 he was called to succeed him in that office. Decisively beaten by the Liberals in 1860, he spent some time in Europe advocating foreign intervention in Mexican affairs; and returned as a partisan of Maximilian. His ability as a soldier was shown by his double defence of Puebla in 1856.  MIRANDA, FRANCESCO (c. 1754–1816), Spanish-American soldier and adventurer, was born at Caracas, Venezuela, about 1754. He entered the army, and served with the French in the American War of Independence. The success of that war inspired him with a belief that the independence of Spanish America would increase its prosperity. He began to scheme a revolution, but was discovered and had only just time to escape to the United States. Thence he went to England, where he was introduced to Pitt, but chiefly lived with the leading members of the opposition—Fox, Sheridan and Romilly. Finding no help, he travelled through Austria and Turkey to Russia, where he was warmly received, but was dismissed with rich presents, at the demand of the Spanish ambassador, backed up by France. The news of the dispute between England and Spain about Nootka Sound in 1790 recalled him to England, where he saw a good deal of Pitt, but the peaceful arrangement of the dispute again destroyed his hopes. In April 1792 he went to Paris, with introductions to Pétion and the leading Girondists, hoping for aid in South America. France had too much to do to help others; but Miranda’s friends sent him to the front as general of brigade. He distinguished himself under Dumouriez, was entrusted in February 1793 with the siege of Maestricht, and commanded the left wing of the French army at the disastrous